Diseases of the aortic valve and aortic root have been treated surgically for many years, but the structure and function of these parts is still imperfectly understood. As a result, valvuloplasty is carried out empirically and not on the basis of detailed knowledge of leaflet strengths and weaknesses, and surgical prostheses are designed with limited knowledge of the best geometric design and functional concept. A new concept of aortic root function has recently been developed at our institution. According to this concept, the normal aortic root is a dynamic structure integrated with the leaflets and functioning as a single unit. Functional tests of the thesis have produced affirmative results but support from structural data has been lacking. The proposed research is designed to demonstrate the functional histology and interrelationships of the constituent tissues in the aortic root and leaflets. The histology of fibrous supporting tissues of the wall and leaflet, including their interdigitations and insertions, will be studied by light microscopy from serial sections of human and animal valves. Details of fiber attachments and identification of small fibrous components will be studied by electron microscopy. The possibility of tissue turnover in valves will be examined with radioisotopes and tissue radioautography.